Various synchronization methods for OFDM signals are known in the prior art, but none of these reaches the levels of high security needed to avoid false indications of synchronization in point to multipoint systems where the means of transmission is the electricity network.
It is well known that the electricity network is a difficult transmission medium due to the fact that the connection and disconnection of different apparatus on the network produces voltage peaks and impedance variation on the line, so that channel response varies over time.
Among the synchronization methods known, one must emphasize the method described in U.S. Pat. No. 5,732,113, concerning a time synchronization process using one synchronization symbol in two equal halves, where, to carry out the synchronization a lesser number of samples is used than the number proposed by the present invention, and where impulse noise, which is very common in the electricity network, affects the synchronization to a much greater degree due to the fact that by definition these are occasional noises affecting a small number of samples, so they will affect this process more than the solution proposed in the present invention, and therefore, the method of using only one synchronization symbol is not desirable in a transmission system using the electricity network.
One must also indicate that the use of one synchronization symbol means greater variance in the calculation or estimation of the synchronization due to the use of a smaller number of samples to carry out the synchronization.
P. Moose published the concept of using two equal symbols in: “A technique for orthogonal frequency division multiplexing frequency offset correction. IEEE Trans. on Comm., vol. 42, pp2908-2914, October 1994”, but these symbols have never been considered or used to carry out time synchronization, as occurs in the present invention, instead, Moose used these to estimate errors in the frequency of analog translation.
Finally, carrying out frequency synchronization by means of the arctangent of the correlation to correct errors in the frequency of analog translation is known in the prior art, for example in U.S. Pat. No. 5,732,113 or in the article by Moose. To carry out frequency synchronization in the current invention involves estimating the sampling frequency error in the analog/digital converters in each one of the user receivers, which is substantially different from what is known in the prior art.